During a normal heartbeat, the heart contracts in a coordinated fashion to pump blood. In particular, the heart contracts based on rhythmic electrical impulses, which are spread over the heart using specialized fibers. These rhythmic electrical pulses are initiated by the heart's natural pacemaker called the sinoatrial node (“SA node”). The SA node initiates electrical impulses to cause the right and left atrium to contract. As the atria contract, the electrical impulses from the SA node propagate to the atrial-ventricular node (“AV node”). After an inherent delay in the AV node, the AV node then transmits the electrical impulses, which eventually causes contraction in the right and left ventricles. The inherent delay of the AV node is known as the A-V delay and allows the atria to fully contract and fill the ventricles with blood. Blood from the ventricles then flows out of the heart and to the rest of the body. Therefore, the heart relies upon a rhythmic cycle of electrical impulses to pump blood efficiently.
A heart may suffer from one or more cardiac defects that interfere with the rhythmic cycle or conduction of electrical impulses. For example, one known heart condition is an AV nodal block. An AV nodal block inhibits transfer of impulses from the SA node to the AV node, and thus, inhibits or prevents contraction of the right and left ventricles. Other conditions, such as myocardial scarring and bundle branch block, may slow conduction of impulses, and thus, cause the heart to beat in an uncoordinated fashion.
In diseased hearts having conduction defects and in congestive heart failure (CHF), cardiac depolarizations that naturally occur in one upper or lower heart chamber are not conducted in a timely fashion either within the heart chamber or to the other upper or lower heart chamber. In such cases, the right and left heart chambers do not contract in optimum synchrony with one another, and cardiac output suffers.
Typically, an artificial pacemaker is installed to treat these and other various cardiac deficiencies. For example, in the case of loss of A-V synchrony, a single chamber, demand pacemaker may sense impulses from the SA node and then supply stimulating electrical pulses to the ventricles to cause contraction in the right and left ventricles. In this manner, an artificial pacemaker may compensate for blocked or slowed conduction of electrical impulses from the atrium to the ventricles in the heart.
Dual chamber, demand pacemakers typically supply pacing pulses when required to one upper heart chamber and to one lower heart chamber, usually the right atrium and the right ventricle. In a dual chamber, demand pacemaker operating in DDD pacing mode, an atrial pacing pulse is delivered to the atrium if an atrial contraction is not sensed within an atrial escape interval (A—A interval) and a ventricular pacing pulse is delivered to the ventricle if a ventricular contraction is not sensed within a ventricular escape interval (V—V interval).
Patients suffering from congestive heart failure and other conduction defects may require bi-ventricular and/or bi-atrial pacing. For example, in a dual chamber bi-atrial pacemaker, the right atrium may be paced at the expiration of an A—A escape interval, and the left atrium is synchronously paced or paced after a short delay. In a dual chamber bi-ventricular pacemaker, the right ventricle may be paced at the expiration of a V—V escape interval, and the left ventricle is synchronously paced or paced after a short delay time. In a single chamber pacemaker with bi-chamber pacing, a pacing pulse delivered at the end of an AV delay may trigger the simultaneous or slightly delayed delivery of the pacing pulse to the other heart chamber.
In order to provide stimulating electrical pulses, known artificial pacemakers may include multiple stimulators. Furthermore, an artificial pacemaker may include multiple stimulators that are triggered at different times to provide dual chamber and/or bi-chamber pacing. Unfortunately, providing and controlling multiple stimulators increases the number of components that may fail within an artificial pacemaker.
Accordingly, it would be desirable to provide methods, apparatus, and systems, which can avoid using multiple stimulators and overcome other deficiencies in the prior art.